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<h3>Solids: Solubility</h3>

<p>
The solubility of oxides, hydroxides or oxihydroxides for
many transition metals is strongly dependent on the particle size and
crystallinity of the solid.
</p>
<p>
For <nobr>Fe(III),</nobr> <nobr>Al(III),</nobr> etc, there is a large difference
between the solubilities of crystalline oxides and amorphous hydroxides. The amorphous
solids precipitate when the oversaturation is high, as when mixing solutions in
the laboratory. The amorphous solids will slowly re-crystallise and form more
insoluble (more stable) solids.
</p>
<p>
The following diagram shows that for <nobr>Fe(III)</nobr> the difference is a factor
of almost <nobr>10<sup>5</sup>:</nobr></p>
<p><center>
<img src="images/Diagr_SolubFe.gif" alt="Diagr_Solub_Fe(III)" title="Diagr_Solub_Fe(III)" height="179" width="268" border="1">
</center>
The endings &#147;(am)&#148; and &#147;(cr)&#148; in the formulas are
<a href="SP_Phase_Indicator_Names.htm">phase indicators</a>.
The diagram shows that <nobr>[Fe(III)]<sub>TOT</sub></nobr> &#8776;
<nobr>10<sup>&#8722;12.3</sup></nobr> when haematite
<nobr>(Fe<sub>2</sub>O<sub>3</sub>(cr))</nobr> is in equilibrium with an aqueous
solution of <nobr>pH =&nbsp;8.</nobr> The diagram also shows that you can have
a <nobr>10<sup>&#8722;5</sup>&nbsp;mM</nobr>
solution of <nobr>Fe(III)</nobr> at <nobr>pH 2</nobr> without precipitation:
the solution is metastable. If the pH is increased, amorphous <nobr>Fe(OH)<sub>3</sub></nobr>
precipitates at pH about <nobr>4.3</nobr>
</p>

<h4>Dissolution or precipitation?</h4>

<p>To model the precipitation of Fe(III), the diagram above shows that
the amorphous hydroxide,
<nobr>Fe(OH)<sub>3</sub>(am),</nobr> should be included in the calcualtions,
but the crystalline phases should be excluded
<nobr>(Fe<sub>2</sub>O<sub>3</sub>(cr)</nobr> and <nobr>FeOOH(cr)).</nobr>
On the other hand, to model a dissolution reaction the dissolving solid must be included
in the calculations.</p>

<p>
To exclude solids, they can be <a href="SP_Removing_Species.htm">deactivated</a>
in the calculations,
see <a href="S_Modify_Chem_System.htm#deactivate">deactivate spcies in SPANA</a>.
Alternatively, you can exclude <nobr>&#147;(cr)&#148;-solids</nobr> from the search in DATABASE.
</p> 

<h4><a href="DB_0_Main.htm">DATABASE</a>: Search preferences for solids</h4>

<p>The menu &#147;Options / Preferences / Solids&#148;
will display a secondary window where the user can choose between:
<ul>
<li><u>Include all solids</u> when searching the databases. This option is the default.
This should be choosen to model processes where solids are added and their
<b>dissolution</b> process is being studied.</li> 
<li><u>Exclude solid</u> names ending with <nobr>&#147;(cr)&#148;.</nobr> This option
should be chosen when modelling <b>precipitation</b> processes.</li>
</ul></p>

<p>See also
<ul><li><a href="DB_Solids.htm">solids in logK files</a>,</li>
<li><a href="ChEq_Solids.htm">chemical equilibria with solids</a>.</li>
</ul></p>
  
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